The present invention relates to a system for oxygen delignification in accordance with the preamble to Patent claim 1.
A number of different processes for oxygen delignification have been disclosed.
U.S. Pat. No. 4,259,150 presents a system involving a multistage oxygen bleaching in which the pulp is, in each stage, firstly mixed to a lower consistency with O2, water and NaOH, followed by a thickening back to the consistency level which the pulp had up until the stage in question. The aim is to achieve an economical, chlorine-free bleaching with high yield. At the same time, the kappa number can be lowered, by means of repeated stages, from 70 down to 15, or even to less than 15.
SE,C,467.582 presents an improved system for the oxygen bleaching of pulp of medium consistency. By means of the temperature control having been optimized, an oxygen bleaching takes place in a first delignification zone at low temperature, followed by a second delignification zone which is at a temperature which is 20-40 degrees higher. The aim was to obtain an improved yield and an improved viscosity, while retaining the same dwell time, in connection with industrial implementation.
In addition to SE,C,467.582, other variants of oxygen delignification in two stages have also been patented. SE,C,505.147 presents a process in which the pulp should have a high pulp concentration, in the range of 25-40%, in the first stage and a concentration of 8-16% in the second stage, at the same time as the temperature in the second stage should be higher than, or the same as, the temperature in the first stage, in line with the temperature difference which is recommended in SE,C,467.582. The advantages of the solution in accordance with SE,C,505.147 are stated to be the possibilities of admixing more oxygen in the first high-consistency stage without the risk of channel formation but where, at the same time, unused quantities of oxygen can be bled off after the first stage for subsequent admixture in a second mixer prior to the second stage.
SE,C,505.141 presents yet another process, which is an attempt to circumvent SE,C,467.582, since that which it is sought to patent is stated to be that the temperature difference between the stages does not exceed 20 degrees, i.e. the lower suitable temperature difference which is patented in SE,C,467.582, but that nevertheless a temperature difference should exist. In addition to this, it is stated that a) the pressure should be higher in the first stage and b) that the dwell time is short in the first stage, i.e. of the order of magnitude of 10-30 minutes, and c) the dwell time in the second stage is longer, i.e. of the order of magnitude of 45-180 minutes.
A lecture entitled xe2x80x9cTwo-stage MC-oxygen delignification process and operating experiencexe2x80x9d, which was given by Shinichiro Kondo, from the Technical Div.
Technical Dept. OJI PAPER CO. Ltd., at the 1992 Pan-Pacific Pulp and Paper Technology Conference (""99 PAN-PAC PPTC), Sept. 8-10, Sheraton Grande Tokyo Bay Hotel and Towers, presents a successful installation which involves two-stage oxygen delignification and which was constructed in 1986 in a plant in Tomakomai.
In this OJI PAPER plant in Tomakomai, the pulp was fed, at a pressure of 10 bar, to a first oxygen mixer (+steam), followed by an aftertreatment in a xe2x80x9cpreretention tubexe2x80x9d (prereactor), with a dwell time of 10 minutes, in which the pulp pressure is reduced to a level of about 8-6 bar due to pipe losses, etc. After that, the pulp was fed to a second oxygen mixer, followed by an aftertreatment in a reactor at a pressure of 5-2 bar and with a dwell time of 60 minutes. At this point, it was stated that preference would have been given to having a xe2x80x9cpreretention tubexe2x80x9d which would have given a dwell time of about 20 minutes but that it was not possible to achieve this due to lack of space. OJI PAPER stated that, by using this installation, they were successful in achieving an increase in kappa reduction for a lower cost in chemicals and also an improvement in pulp viscosity.
The greater part of the prior art has consequently been directed towards a higher pressure in the first reactor to a level of about 6(8)-10 bar. A pressure in the first reactor of up to 20 bar has even been discussed in some extreme applications. This entails the reactor spaces which are required for the first delignification zone having to be manufactured so as to cope with these high pressure levels, with the attendant requirement for substantial material thickness and/or good material qualities, resulting in an expensive installation.
Conventionally, these reactors are used as upright vessels, with the pulp flowing either upstream or downstream through the reactor. A problem then is that disparate reaction conditions arise through the reactor since the pressure changes during the process. When a vertical reactor having a height of 10 meters is used, a difference in pressure is then obtained simply due to the hydrostatic effect of 1 bar. As a result, the delignification process cannot be optimized equally well with regard to the pressure.
In pulp suspensions used in industrial manufacturing processes, there are large quantities of readily oxidizable constituents/structures which react even under modest process conditions. It is therefore advantageous to add oxygen in a first stage in quantities which are such that this relatively easily oxidized part of the pulp is allowed to oxidize/react first of all. Severe problems arise if an attempt is made to compensate for this by overadding oxygen since there is the imminent danger of channelling problems (as mentioned in the said SE,C,505.147).
One object of the invention is to avoid the disadvantages of the prior art and to obtain an oxygen delignification of increased selectivity. The invention permits an optimal practical application of the theories regarding a first rapid phase and a second slower phase during the oxygen delignification process, where the optimal reaction conditions are different between the phases.
At the high hydroxide ion concentrations and high oxygen partial pressures which are conventionally employed in the first stage, the carbohydrates are attacked more than necessary, thereby impairing the quality of the pulp. A lower oxygen partial pressure, and preferably also a lower temperature, in the first stage than in the second stage decreases the rate of reaction for breaking down carbohydrates more than it decreases the rate of reaction for the delignification, thereby leading to an increased total selectivity on the pulp after the two stages.
Another object is to permit a simpler and cheaper process installation, in which it is possible to manufacture at least one pressure vessel in a first delignification zone using less robust material and/or a lower material quality which is suitable for a lower pressure class.
Yet another object is to permit an additional simpler and cheaper process installation in which the first pulp-conveying pump can be of a simple type which is dimensioned only for transporting the pulp through the first delignification zone. The process installation can also be effected in delignification plants in which the stations for adding oxygen are located very close to each other. Normally, an attempt is made to keep stations for adding oxygen and adding chemicals within a restricted area in order to limit working environment problems and discharge risks.
Yet another object is to make it possible also to use steam at moderate pressure, especially when there is a need to increase the temperature substantially between the first and second stages and where the pressure in the second stage is considerably higher than that the first stage. This makes it possible to convert existing single-vessel delignification systems where, with the previously known technique for converting to a two-stage design, a limitation has been that the prevailing pressure in the plant""s steam grid has not enabled a sufficiently large quantity of steam to be admixed in the pulp in order to achieve the desired temperature in the second delignification stage.
Yet another object is to optimize the mixing process in each position such that only that quantity of oxygen is added which is consumed in the subsequent delignification zone. This makes it possible to dispense with bleeding systems for overshooting quantities of oxygen at the same time as it is possible to reduce the total consumption of oxygen, thereby reducing the operating costs for the operator of the fibre line and thus shortening the pay-off time.
Yet another object is to increase, in an oxygen delignification system having a given total volume of the first and second stages, a so-called H factor by operating the first stage for a short time at a low temperature and the second stage for a longer time at a higher temperature. When, for example, carrying out conversions of existing single-vessel oxygen delignification stages, a simple conversion, with a smaller prereactor and a modest increase in the reaction temperature in the existing reactor, can increase the H factor and at the same time improve the selectivity over the oxygen stages.
The invention is described in more detail with reference to the figures in accordance with the following figure list.